This invention concerns improvements in or relating to optical apparatus and relates more particularly to optical viewing apparatus of the type having an intermediate image detector.
In such apparatus radiation from a scene or object under view is focussed by an objective lens to form an image of the scene or object which is detected by the detector. A display of the detected image is then produced for viewing through an eye-piece lens, sometimes otherwise referred to as a magnifier. The radiation may be invisible and the apparatus may comprise in conjunction with the detector a convertor effective to produce a visible display of the detected invisible image. Alternatively, the radiation may be visible and the apparatus may comprise in conjunction with the detector an intensifier or amplifier effective to produce a display of increased brightness of the detected visible image. The display image viewed by the observer needs to be of adequate optical quality and the optics of the apparatus are designed to achieve this. For example, if necessary the objective lens may introduce a little barrel distortion to counteract inherent pincushion distortion of the eye-piece lens or magnifier, or may, as described in British Patent Specification No. 1,569,429,, produce barrel distortion to counter electronically induced pincushion distortion of an image intensifier tube.
Problems can be encountered with optical viewing apparatus of the above type by reason of limited resolving power of the detector. Such a problem is discussed in the context of night-vision apparatus employing image intensifiers in British Patent Specification No. 1,524,691 which proposes superimposing at the same magnification two separately generated intensified images of a scene, one image providing a comparatively wide-angle view of the scene and the other image providinga comparatively narrow-angle view of the scene but with better resolution.